1. Field of the Invention
This invention generally relates to balanced output filters, and more particularly, to a balanced output filter excellent in both amplitude balance characteristics and phase balance characteristics.
2. Description of the Related Art
An interstage filter used for a receive circuit of a mobile telephone terminal often employs a balanced output filter that can reduce noises and can be connected to an external IC directly. For example, the balanced output filter utilizing a surface acoustic wave (hereinafter referred to as SAW) is capable of realizing low insertion loss and high suppression at the same time. Therefore, in most cases, the aforementioned balanced filters are incorporated into the mobile telephone terminals distributed in the market. The structure of the balanced filter is classified into several categories, one of which is a “parallel balanced filter”.
FIG. 1 is a schematic view showing a representative structure of the parallel balanced filter. An input terminal 102a of a filter 101a is connected to another input terminal 102b of another filter 101b. The input terminals 102a and 102b are connected to an unbalanced external input terminal 104. An output terminal 103a of the filter 101a and an output terminal 103b of the filter 101b are balanced output terminals. One of the filters is an inphase filter (101a), and is designed so that phases of an input signal and an output signal are substantially equal (inphase) ground the center frequency. The other filter is an antiphase filter (101b), and is designed so that the phases of the input signal and the output signal are substantially opposite (antiphase) by 180 degrees ground the center frequency. Balanced outputs are the outputs available at the output terminal 103a (a first balanced output terminal) of the inphase filter 101a and at the output terminal 103b (a second balanced output terminal) of the antiphase filter 101b. Another structure of the parallel balanced filter may include multiple inphase filters and antiphase filters and the input terminals and output terminals thereof are connected each other.
The low insertion loss in a passband and the high suppression in a stopband are especially important functions of the characteristics of the balanced filter. In addition, amplitude balance and phase balance are also important. The characteristics of the amplitude balance and those of the phase balance are simply referred to as balance characteristics. The amplitude balance is defined by a difference between one signal amplitude output from the output terminal (the first balanced output terminal) of the inphase filter and the other signal amplitude output from the output terminal (the second balanced output terminal) of the antiphase filter. The amplitude balance is considered more excellent as the difference is closer to zero. dB is usually used for a unit of the amplitude balance. On the other hand, the phase balance is defined by a difference between one signal phase output from the first balanced output terminal and the other signal phase output from the second balanced output terminal. The phase balance is considered more excellent as the difference is closer to 180 degrees. The above-mentioned balance characteristics have a dependency on the frequency. The balance characteristics are evaluated by how the amplitude balance is close to 0 dB, and how the phase balance is close to 180 degrees, in the passband of the balanced output filter.
FIG. 2 shows an example of a structure of a parallel balanced filter having a SAW filter thereon. An inphase filter and an antiphase filter included in the parallel balanced filter are double-mode SAW (hereinafter referred to as DMS) filters. It is easy to selectively use inphase or antiphase in the DMS filter, and the DMS filter is often used for the balanced filter. Here, the inphase filter is also referred to as inphase DMS, and the antiphase filter is also referred to as antiphase DMS.
An inphase DMS 201a and an antiphase DMS 201b are provided on a piezoelectric substrate 200. The inphase DMS 201a includes an input IDT (interdigital transducer) 205a, output IDTs 206a1 and 206a2 respectively arranged on both sides of the input IDT 205a. A pair of reflectors 207a1 and 207a2 are arranged outer sides of the output IDTs 206a1 and 206a2 respectively. An input ground terminal 209a is connected to ground electrode fingers of the input IDT 205a. Output ground terminals 208a1 and 208a2 are connected to ground electrode fingers of the output IDTs 206a1 and 206a2. In the same manner, the antiphase DMS 201b includes an input IDT (interdigital transducer) 205b, output IDTs 206b1 and 206b2 respectively arranged on both sides of the input IDT 205b. A pair of reflectors 207b1 and 207b2 is arranged outer sides of the output IDTs 206b1 and 206b2 respectively. An input ground terminal 209b is connected to ground electrode fingers of the input IDT 205b. Output ground terminals 208b1 and 208b2 are connected to ground electrode fingers of the output IDTs 206b1 and 206b2. Signals input from an unbalanced external input terminal 204 are input into the inphase DMS 201a and the antiphase DMS 201b through the respectively provided input terminals 202a and 202b, and are output from balanced output terminals 203a and 203b. 
It is possible to selectively connect the inphase DMS or the antiphase DMS in the above-mentioned parallel balanced filter, by reversing the polarities of the electrode fingers of the input IDT or the output IDT, changing the number of the electrode fingers of the input IDT, or changing gaps between the input IDT and the output IDT. The above-mentioned SAW filter chip is housed in a ceramic package or a plastic package. The SAW filter chip is electrically coupled to the package by bonding wires, Au bumps, or solder balls. A facedown structure is mainly employed in these years, because the facedown structure makes it possible to downsize and reduce the thickness of a whole SAW device.
FIGS. 3A and 3B are views of a conventional balanced output SAW filter having the SAW filter chip facedown bonded to a package 210. The SAW chip is formed on the piezoelectric substrate 200. FIG. 3A illustrates each component in detail to mount the SAW filter chip on the package. FIG. 3B is a top view of a die attach surface of the package. This package is composed of a laminated body of a first layer arranged as a bottom layer and a second layer arranged as a top layer. The die attach surface forms an upper surface of the first layer. The SAW filter chip is made up of the inphase DMS 201a and the antiphase DMS 201b. The inphase DMS 201a and the antiphase DMS 201b are connected in parallel on the piezoelectric substrate 200. The inphase DMS 201a includes the input terminal 202a, the input ground terminal 209a, the output ground terminals 208a1 and 208a2, and the balanced output terminal 203a. The antiphase DMS 201b includes the input terminal 202b, the input ground terminal 209b, the output ground terminals 208b1 and 208b2, and the balanced output terminal 203b. The input terminals 202a and 202b are connected to the external input terminal 204 provided on the piezoelectric substrate 200.
Referring to FIG. 3B, the die attach surface of the package 210 (a mounting surface of the SAW filter chip) has an input terminal 211, a ground terminal 212, a first balanced output terminal 213a, and a second balanced output terminal 213b. Au bumps (or solder balls) 214a through 214i are provided on the filter chip so as to bond and connect the aforementioned terminals and the package 210. The Au bump 214a provided on the input terminal 211 is coupled to the input terminal 204 of the SAW filter chip. The Au bump 214b provided on the first balanced output terminal 213a is coupled to the balanced output terminal 203a of the inphase DMS 201a of the SAW filter chip. The Au bump 214c provided on the second balanced output terminal 213b is coupled to the balanced output terminal 203b of the antiphase DMS 201b of the SAW filter chip. The Au bumps 214d through 214i provided on the ground terminal 212 are divided into two groups. Four Au bumps 214d through 214g are coupled to the output ground terminals 208a1, 208a2, 208b1, and 208b2 of the SAW filter chip. Two Au bumps 214h and 214i are connected to the input ground terminals 209a and 209b. 
That is to say, the input ground terminals 209a and 209b and the output ground terminals 208a1, 208a2, 208b1, and 208b2 are connected on a metal pattern on the die attach surface of the package 210, and then are connected to a first ground footpad 216a and a second ground footpad 216b through a first via 215a and a second via 215b. The first ground footpad 216a and the second ground footpad 216b are provided on a backside of the package 210. The input terminal 211, the first balanced output terminal 213a, and the second balanced output terminal 213b provided on the die attach surface are respectively connected to the corresponding footpads through a third via 215c, a fourth via 215d, and a fifth via 215e. The corresponding footpads are an input footpad 217, a first balanced output footpad 218a, and a second balanced output footpad 218b. 
FIGS. 4A and 4B are graphs showing examples of the balance characteristics of the balanced output SAW filter having the above-mentioned structure. FIG. 4A shows amplitude balance characteristics. FIG. 4B shows phase balance characteristics. In both FIGS. 4A and 4B, the vertical axis denotes frequency (MHz). The horizontal axis in FIG. 4A denotes the amplitude balance by the dB. The horizontal axis in FIG. 4B denotes the phase balance by the degree. These figures tell that the amplitude balance is ±0.9 dB or less, and the phase balance is 180±5 degrees, when the balance characteristics are evaluated.
Japanese Patent Application Publication No. 2003-273707 discloses an elastic wave device having balancing capabilities. This elastic wave device is not connected to an external impedance chip to the outside, and is excellent in the amplitude balance in the passband in particular. Japanese Patent Application Publication No. 9-116377 discloses an elastic wave device having a packaged structure that is capable of obtaining high attenuation characteristics. However, more excellent balance characteristics are demanded for the balanced output filter used on a today's mobile telephone terminal in order to increase the flexibility of the circuit design in the mobile telephone terminal. The amplitude balance is demanded to be closer to 0 dB, and the phase balance is demanded to be closer to 180 degrees.